I reiterate what Len mentioned.

You need a properly functioning set of glow plugs, valves set to proper lash, and a new (less than three year old) battery of the proper size to start an older diesel at cold temps.

Compression also is an issue. If the engine is marginal on compression, even all of the above won't allow a start if the temp drops very low. "Very low" is relative to the compression in the engine.

If the compression is low and all of the above items are checked and verified, the only recourse if you wish to keep the vehicle is to install a diesel fired block heater for a cost of about $800. or so. The engine will start at any temperature.